Late Cenozoic tephrostratigraphy offshore the southern Central American Volcanic Arc: 1. Tephra ages and provenance

We studied the tephra inventory of 18 deep sea drill sites from six DSDP/ODP legs (Legs 84, 138, 170, 202, 205, 206) and two IODP legs (Legs 334 and 344) offshore the southern Central American Volcanic Arc (CAVA). Eight drill sites are located on the incoming Cocos plate and ten drill sites on the continental slope of the Caribbean plate. In total we examined ∼840 ash-bearing horizons and identified ∼650 of these as primary ash beds of which 430 originated from the CAVA. Correlations of ash beds were established between marine cores and with terrestrial tephra deposits, using major and trace element glass compositions with respect to relative stratigraphic order. As a prerequisite for marine-terrestrial correlations we present a new geochemical data set for significant Neogene and Quaternary Costa Rican tephras. Moreover, new Ar/Ar ages for marine tephras have been determined and marine ash beds are also dated using the pelagic sedimentation rates. The resulting correlations and provenance analyses build a tephrochronostratigraphic framework for Costa Rica and Nicaragua that covers the last >8 Myr. We define 39 correlations of marine ash beds to specific tephra formations in Costa Rica and Nicaragua; from the 4.15 Ma Lower Sandillal Ignimbrite to the 3.5 ka Rincón de la Vieja Tephra from Costa Rica, as well as another 32 widely distributed tephra layers for which their specific region of origin along Costa Rica and Nicaragua can be constrained

The Masaya Triple Layer: a 2100 year old basaltic multi-episodic Plinian eruption from the Masaya Caldera Complex (Nicaragua)

The Masaya Caldera Complex has been the site of three highly explosive basaltic eruptions within the last six thousand years. A Plinian eruption ca. 2 ka ago formed the widespread deposits of the Masaya Triple Layer. We distinguish two facies within the Masaya Triple Layer from each other: La Concepción facies to the south and Managua facies to the northwest. These two facies were previously treated as two separated deposits (La Concepción Tephra and the Masaya Triple Layer of Pérez and Freundt, 2006) because of their distinct regional distribution and internal architectures. However, chemical compositions of bulk rock, matrix and inclusion glasses and mineral phases demonstrate that they are the product of a single basaltic magma batch. Additionally, a marker bed containing fluidal-shaped vesicular lapilli allowed us to make a plausible correlation between the two facies, also supported by consistent lateral changes in lithologic structure and composition, thickness and grain size. We distinguish 10 main subunits of the Masaya Triple Layer (I to X), with bulk volumes ranging between 0.02 and 0.22 km3, adding up to 0.86 km3 (0.4 km3 DRE) for the entire deposit. Distal deposits identified in two cores drilled offshore Nicaragua, at a distance of ∼ 170 km from the Masaya Caldera Complex, increase the total tephra volume to 3.4 km3 or ∼ 1.8 km3 DRE of erupted basaltic magma. Isopleth data of five major fallout subunits indicate mass discharges of 106 to 108 kg/s and eruption columns of 21 to 32 km height, affected by wind speeds of < 2 m/s to ∼ 20 m/s which increased during the course of the multi-episodic eruption. Magmatic Plinian events alternated with phreatoplinian eruptions and phreatomagmatic explosions generating surges that typically preceded breaks in activity. While single eruptive episodes lasted for few hours, the entire eruption probable lasted weeks to months. This is indicated by changes in atmospheric conditions and ash-layer surfaces that had become modified during the breaks in activity. The Masaya Triple Layer has allowed to reconstruct in detail how a basaltic Plinian eruption develops in terms of duration, episodicity, and variable access of external water to the conduit, with implications for volcanic hazard assessment

Stratigraphy of the Pleistocene, phonolitic Cão Grande Formation on Santo Antão, Cape Verde

Highlights: • Two new phonolitic tephra units complementing the two previously known. • First radiometric ages of the CGF. • Contemporaneously evolution of the CGF and the Tope de Coroa. • Marine correlations improve tephra volume estimations for CG I and II. Abstract: The Cão Grande Formation (CGF) on the western plateau of Santo Antão Island is part of the younger volcanic sequence that originated from both, basanitic and nephelinitic magmatic suites, respectively called COVA and COROA suites. Based on our detailed revised stratigraphy of the CGF, including two yet unknown tephra units, we can show that both suites produced multiple, highly differentiated eruptions over a contemporaneous period. Correlations of CGF tephras with marine ash layers provide distal dispersal data for Cão Grande I (CG I) and also identify two highly explosive, phonolitic eruptions that pre-date the CGF tephra deposits known on land. Within the CGF, the lowermost, 220±7 ka old unit Canudo Tephra (CT; COVA suite) comprises phonolitic fall deposits and ignimbrites; it is partly eroded and overlain by debris flow deposits marking a hiatus in highly differentiated eruptions. The phonolitic CG I Tephra (COROA suite) consists of an initial major plinian fall deposit and associated ignimbrite and terminal surge deposits. This is immediately overlain by the phonolitic to phono-tephritic Cão Grande II (CG II; COVA suite), a complex succession of numerous fallout layers and density-current deposits. CG I and CG II have radiometric ages of 106±3 ka and 107±15 ka, respectively, that are identical within their error limits. The youngest CGF unit, the Furninha Tephra (FT; COROA suite), consists of three foidic-phonolitic fall deposits interbedded with proximal scoria deposits from a different vent. The phonolitic eruptions switched to and fro between both magmatic suites, in each case with a stronger first followed by a weaker second eruption. Each eruption evolved from stable to unstable eruption columns. During their terminal phases, both magma systems also leaked evolved dome-forming lavas next to the tephras. Distal ashes increase the CG I tephra volume to ~ 10 km3, about twice the previously published estimate. The tephra volume of CG II is ~ 3 km3; CT and FT are too poorly exposed for volume estimation. The characteristics of the CGF tephra units outline hazard conditions that may be expected from future evolved explosive eruptions on the western plateau of Santo Antão

Late Cenozoic tephrostratigraphy offshore the southern Central American Volcanic Arc: 2. Implications for magma production rates and subduction erosion

Pacific drill sites offshore Central America provide the unique opportunity to study the evolution of large explosive volcanism and the geotectonic evolution of the continental margin back into the Neogene. The temporal distribution of tephra layers established by tephrochonostratigraphy in Part 1 indicates a nearly continuous highly explosive eruption record for the Costa Rican and the Nicaraguan volcanic arc within the last 8 M.y. The widely distributed marine tephra layers comprise the major fraction of the respective erupted tephra volumes and masses thus providing insights into regional and temporal variations of large-magnitude explosive eruptions along the southern Central American Volcanic Arc (CAVA). We observe three pulses of enhanced explosive magmatism between 0-1 Ma at the Cordillera Central, between 1-2 Ma at the Guanacaste and at >3 Ma at the Western Nicaragua segments. Averaged over the long-term the minimum erupted magma flux (per unit arc length) is ∼0.017 g/ms. Tephra ages, constrained by Ar-Ar dating and by correlation with dated terrestrial tephras, yield time-variable accumulation rates of the intercalated pelagic sediments with four prominent phases of peak sedimentation rates that relate to tectonic processes of subduction erosion. The peak rate at >2.3 Ma near Osa particularly relates to initial Cocos Ridge subduction which began at 2.91±0.23 Ma as inferred by the 1.5 M.y. delayed appearance of the OIB geochemical signal in tephras from Barva volcano at 1.42 Ma. Subsequent tectonic re-arrangements probably involved crustal extension on the Guanacaste segment that favored the 2-1 Ma period of unusually massive rhyolite production

Silicate weathering in anoxic marine sediments

Two sediment cores retrieved at the northern slope of Sakhalin Island, Sea of Okhotsk, were analyzed for biogenic opal, organic carbon, carbonate, sulfur, major element concentrations, mineral contents, and dissolved substances including nutrients, sulfate, methane, major cations, humic substances, and total alkalinity. Down-core trends in mineral abundance suggest that plagioclase feldspars and other reactive silicate phases (olivine, pyroxene, volcanic ash) are transformed into smectite in the methanogenic sediment sections. The element ratios Na/Al, Mg/Al, and Ca/Al in the solid phase decrease with sediment depth indicating a loss of mobile cations with depth and producing a significant down-core increase in the chemical index of alteration. Pore waters separated from the sediment cores are highly enriched in dissolved magnesium, total alkalinity, humic substances, and boron. The high contents of dissolved organic carbon in the deeper methanogenic sediment sections (50–150 mg dm−3) may promote the dissolution of silicate phases through complexation of Al3+ and other structure-building cations. A non-steady state transport-reaction model was developed and applied to evaluate the down-core trends observed in the solid and dissolved phases. Dissolved Mg and total alkalinity were used to track the in-situ rates of marine silicate weathering since thermodynamic equilibrium calculations showed that these tracers are not affected by ion exchange processes with sediment surfaces. The modeling showed that silicate weathering is limited to the deeper methanogenic sediment section whereas reverse weathering was the dominant process in the overlying surface sediments. Depth-integrated rates of marine silicate weathering in methanogenic sediments derived from the model (81.4–99.2 mmol CO2 m−2 year−1) are lower than the marine weathering rates calculated from the solid phase data (198–245 mmol CO2 m−2 year−1) suggesting a decrease in marine weathering over time. The production of CO2 through reverse weathering in surface sediments (4.22–15.0 mmol CO2 m−2 year−1) is about one order of magnitude smaller than the weathering-induced CO2 consumption in the underlying sediments. The evaluation of pore water data from other continental margin sites shows that silicate weathering is a common process in methanogenic sediments. The global rate of CO2 consumption through marine silicate weathering estimated here as 5–20 Tmol CO2 year−1 is as high as the global rate of continental silicate weathering

Walking through volcanic mud : the 2,100 year-old Acahualina footprints (Nicaragua) II: the Acahualina people, environmental conditions and motivation

We analyzed bare human footprints in Holocene tuff preserved in two pits in the Acahualinca barrio in the northern outskirts of Managua (Nicaragua). Lithology, volcanology, and age of the deposits are discussed in a companion paper (Schmincke et al. Bull Volcanol doi: 10.1007/s00445-008-0235-9, 2008). The footprint layer occurs within a series of rapidly accumulated basaltic–andesitic tephra that is regionally correlated to the Masaya Triple Layer Tephra. The people were probably trying to escape from a powerful volcanic eruption at Masaya Caldera 20 km farther south that occurred at 2.1 ka BP. We subdivided the swath of footprints, up to 5.6 m wide, in the northern pit (Pit I) into (1) a central group of footprints made by about six individuals, the total number being difficult to determine because people walked in each other’s footsteps one behind the other and (2) two marginal groups on either side of the central group with more widely spaced tracks. The western band comprises tracks of three adjacent individuals and an isolated single footprint farther out. The eastern marginal area comprises an inner band of deep footprints made by three individuals and, farther out, three clearly separated individuals. We estimate the total number of people as 15–16. In the southern narrow and smaller pit (Pit II), we recognize tracks of ca. 12 individuals, no doubt made by the same group. The group represented in both pits probably comprised male and female adults, teenagers and children based on differences in length of footprints and of strides and depth of footprints made in the soft wet ash. The smallest footprints (probably made by children) occur in the central group, where protection was most effective. The footprint layer is composed of a lower 5–15-cm thick, coarse-grained vesicle tuff capped by a medium to fine-grained tuff up to 3 cm thick. The surface on which the people walked was muddy, and the soft ash was squeezed up on the sides of the foot imprints and between toes. Especially, deep footprints are mainly due to local thickening of the water-rich ash, multiple track use, and differences in weight of individuals. The excellent preservation of the footprints, ubiquitous mudcracks, sharp and well-preserved squeeze-ups along the margins of the tracks and toe imprints, and the absence of raindrop impressions all suggest that the eruption occurred during the dry season. The people walked at a brisk pace, as judged from the tight orientation of the swath and the length of the strides. The directions of a major erosional channel in the overlying deposits that probably debouched into Lake Managua and the band of footprints are strictly parallel, indicating that people walked together in stride along the eastern margin of a channel straight toward the lake shore, possibly a site with huts and/or boats for protection and/or escape

Expedition 398 Preliminary Report: Hellenic Arc Volcanic Field

The objectives of International Ocean Discovery Program (IODP) Expedition 398, Hellenic Arc Volcanic Field (11 December 2022 to 10 February 2023), were to study the volcanic record of the central Hellenic island arc; document the links and feedbacks between volcanism/magmatism, crustal tectonics, and sea level; investigate the processes and products of shallow submarine eruptions of silicic magma; and groundtruth the seismic stratigraphy of Santorini caldera. Reconstructing the subsidence history of the southern Aegean Sea and searching for deep life inside and outside of Santorini caldera were additional objectives.The expedition drilled 10 primary and alternate sites that were originally proposed, in addition to 2 extra sites that were requested during the expedition. Outside of Santorini caldera, drilling penetrated the thick basin fills of the crustal rift system hosting the Christiana-Santorini-Kolumbo volcanic field, identifying numerous pumice and ash layers, some known from on land and others hitherto unknown, pushing back the onset of volcanism in the area into the Early Pleistocene or even Pliocene. Significant events of mass wasting into the basins, accompanied by very high sedimentation rates, were also documented. These basin sites served to groundtruth the seismic stratigraphy of the basins and to open the way to unraveling relationships between volcanic activity and crustal rift pulses. Two sites of condensed sequences on the basin margins served to sample many volcanic layers within the detailed age-depth constraints provided mainly by biostratigraphy, as diagenetic effects complicated the magnetic reversal record significantly. Drilling penetrated the Alpine basement at three basin sites northeast of Santorini, whereas in the Christiana Basin to the southwest it penetrated a thick sequence of Messinian evaporites. Drilling inside Santorini caldera penetrated to ~120 meters below seafloor (mbsf), less than planned due to hole instability issues but deep enough to groundtruth the seismic stratigraphy and to sample the different layers. One intracaldera hole yielded a detailed tephra record of the history of the Kameni Islands, as well as possible evidence for deep bacterial colonies within the caldera. Despite variable recovery in the unstable pumice and ash deposits, the expedition was a significant success that may address almost all the science objectives once the laboratory work has been done.A dense program of preexpedition and shipboard outreach during the expedition gave rise to 59 live ship-to-shore tours, reaching 6,400 people in 7 countries including many school children. A total of 51 journalists were contacted and 9 stories were written about the expedition, with a readership of almost 200,000 people. While in Santorini caldera, the ship hosted 12 documentarians and journalists, the future products of whom should include a 1.5 h documentary and a four-part TV series about Expedition 398. The expedition social media pages were active. Prior to the expedition, an exhibition, “In Search of Earth’s Secrets,” ran for a week on Santorini and was visited by more than 1,800 school children

Unraveling the Holocene Eruptive History of Flores Island (Azores) Through the Analysis of Lacustrine Sedimentary Records

Lacustrine sequences from active volcanic settings usually hold a rich and continuous record of tephra layers, providing a critical source of information to reconstruct a most complete eruptive history of a region. Lake sedimentary records on volcanic islands are particularly useful as the typical small size of these islands and their steep subaerial and submarine slopes lead to a lower preservation of potential erodible pyroclastic deposits. Here we explore the lacustrine sedimentary record of Lagoa da Lomba, a crater lake in the central upland area of Flores Island (Azores), to gain insight into the recent eruptive history of this island. The strategic location of Lagoa da Lomba, half distance between the two clusters of recent volcanic activity of the island, together with its long-lasting record, back to 23.52 cal kyr BP, makes this lake a privileged site to investigate the Holocene volcanic history of Flores. Based on a detailed stratigraphic characterization of sediments from a lake transect of three cores, supported by glass shard geochemistry and radiocarbon dating, we recognized four Holocene eruptive events taking place between 6.28 and 2.36 cal kyr BP, demonstrating that the Holocene volcanic activity at Flores Island may have lasted longer than previously reported. Glass shard geochemistry from the different tephra layers suggests three populations, basaltic to trachybasaltic in composition, where the last eruption is the least evolved endmember. Two of the four eruptive events correlate with subaerially-exposed pyroclastic sequences, in terms of stratigraphy and geochemistry. The most recent event recorded at Lagoa da Lomba was constrained to 3.66 - 2.36 cal kyr BP and linked to an eruption sourced from Lagoa Comprida Volcanic System. The second most recent eruptive event was sourced from Lagoa Funda Volcanic System and dated at 3.66 cal kyr BP. Our observations show that Flores experienced vigorous volcanic activity during the Late Holocene. Therefore, contrary to what is assumed, the possibility of future eruptions should be properly considered, and the volcanic hazard here should not be underestimated. Moreover, we highlight the importance of tephrostratigraphy in recent lake sediments to reconstruct past volcanic activity, especially at small volcanic islands, such as Flores, where exposure is poor due to erosion within the limited subaerial area and the dense vegetation